This invention relates in general to apparatus for the disposal of solid waste and the recovery of valuable resources therefrom, and more particularly to an improved vertical shaft furnace or converter for the pyrolysis of refuse.
In an effort to solve ecological problems caused by prior methods of disposing of solid waste, and a desire to recover as much as possible of the natural resources contained in the waste, a process, described and claimed in U.S. Pat. No. 3,729,298, was developed by Anderson. In summary, the Anderson process comprises feeding refuse into the top and oxygen into the base of a vertical shaft furnace. The furnace (or converter) is best described in terms of having three functional zones; a drying zone at the top, a thermal decomposition or pyrolysis zone in the middle, and a combustion and melting zone (or hearth) at the base. It is to be understood that these functional zones are not clearly distinct. That is, there is no sharp line separating them and they may move somewhat during operation. As the refuse descends in the furnace, it is first dried by the hot gas which rise through the furnace, and then pyrolyzed. Pyrolysis is a process whereby organic matter in the refuse is decomposed and thermally cracked in an oxygen-deficient atmosphere with the generation of a CO, H.sub.2 and a char like material. As the refuse moves down through the pyrolysis zone, it is converted in part to volatile materials which rise, and in part to char which descends into the hearth. There it is combusted with oxygen, causing the generation of carbon monoxide, carbon dioxide and the heat required to melt the inorganic solids in the refuse, such as glass and metal. The resulting molten "slag" is continuously tapped from the hearth and quenched in a water tank. A gas mixture containing at least 50% CO and H.sub.2 (on a dry basis) is discharged from the top of the furnace. Following cleanup, the gas may be used as a medium BTU fuel gas or as raw material for chemical synthesis. Apparatus suitable for carrying out the Anderson process described above is shown in U.S. Pat. No. 3,801,082, the disclosure of which is incorporated herein by reference.
It has been found that to efficiently and continuously process shredded refuse through a shaft furnace, it is necessary to form pellets from the refuse before feeding it into the furnace in order to prevent the shredded refuse from compacting so tightly as to unduly restrict the flow of gases through the shaft. The specific characteristics of the refuse pellets required for smooth, long term operation are described in Anderson's copending commonly assigned U.S. application Ser. No. 675,935 filed Apr. 12, 1976 now U.S. Pat. No. 4,042,345, the disclosure of which is incorporated herein by reference. Suitable pellets are characterized by Anderson as having:
1. a density greater than that given by the equation: EQU D = (2,000/(100-0.8A))
where:
D = the density of the pellet (lbs/cu..sup.3), PA1 A = percent inorganics in the refuse pellet, and PA1 R = the ratio of the surface area to the volume of the pellet (ft..sup.2 /ft..sup.3) PA1 H = the height of the refuse bed in the furnace (ft.) PA1 G = the refuse feed rate (tons/day/ft..sup.2 of furnace cross-sectional area).
2. a surface to volume ratio greater than that given by the equation: EQU R = 15(G/H).sup.0.625
where:
It has been found that when operating shaft furnaces of the type described in U.S. Pat. No. 3,801,082 with a pelletized refuse feed, sudden upsets or fluctuations beyond normal operating limits tend to arise periodically in the operating characteristics of the furnace. These upsets are believed to be caused by periodic collapse of the refuse bed above the hearth and lack of adequate gas mixing and distribution in the furnace.
In order to overcome the above problems, it was decided to provide means to support the bed of refuse pellets in the shaft furnace. Such support means must, however, be capable of withstanding the high temperature of the hearth (approximately 3000.degree. F), the highly oxidizing conditions in the base of the furnace, as well as the varying composition of the refuse which causes varying products to be produced. These represent a harsh, corrosive environment to the support means.
Bed support structures are known to be used in coal gasification furnaces as well as in gas cupola furnaces.
Secord, in U.S. Pat. No. 3,253,906 discloses the use of water cooled grates in a coal gasification furnace, wherein a bed of lump bituminous coal is supported on a grate comprising hollow steel pipes extending transversely across the furnace through which a cooling medium is passed. Combustion of the coal takes place above the grate, with the solids remaining on the top of the grate and only the molten products produced being permitted to flow through it.
The use of a supporting grid formed by horizontal hollow steel bars covered with a refractory material is also disclosed in the cupola oxygen furnace shown by Taft et al in U.S. Pat. No. 3,802,678. Resting on these bars is a bed of spherical refractory balls on which the furnace charge rests. The balls permit the passage of gas and provide sufficient length-of-path and time-of-contact for the descending metal charge to melt and become superheated, so that droplets of metal fall through the supporting grid and collect in a pool in the base of the shaft furnace.
The support structures shown in the above-mentioned prior art patents function to prohibit the passage of solid objects through them, allowing only the molten metal to flow through into the hearth. Such prior art grate structures are unsatisfactory for use in shaft furnaces for the pyrolysis of refuse pellets, since here it is necessary that lumps of char formed from the pellets of refuse in the pyrolysis zone pass through the grate into the hearth where the char is combusted to produce the heat necessary to fluidize the inorganic material, as well as producing sufficient heat and to pyrolyze the organic material in the refuse. At the same time, the pellets of unpyrolyzed refuse should not be permitted to fall into the hearth, since if they did, they would upset the operation of the furnace.